1. Field of the Invention
This invention relates to a method of brazing desired parts of an aluminum material which comprises forming a flux layer composed of potassium pentafluoroaluminate (K.sub.2 AlF.sub.5) on the surface of the aluminum material and thereafter heating.
2. Description of the Prior Art
Recently, aluminum material or an aluminum alloy material (hereinafter referred to as the aluminum material) has been used in heat exchangers, such as radiators for automobiles, due to the necessity of reducing the weight. In order to process the aluminum material into products, such as radiators etc., it is at least necessary to join said material with each other such material. The most suitable joining method is brazing because of good operational efficiency and the obtained joined form.
Heretofore, for brazing the aluminum material, there has mainly been used an Al -Si eutectic alloy having a melting point slightly lower than that of the aluminum material as a brazing alloy. Further, in order that the brazing alloy be well joined with the aluminum material, it is necessary to remove oxide films present on the surface of the aluminum material. In order to remove such films, a flux is applied to brazing parts together with the brazing alloy.
A most frequently used flux material is a mixture of metal chlorides such as ZnC1.sub.2, NaCl, etc., containing fluorides. However, when a flux of said chloride substances is used, the flux remaining on the brazed part after a brazing operation can sometimes corrode the aluminum material or the brazing material. Therefore, it was necessary to remove the flux by washing the product after brazing.
On the other hand, it is known that potassium tetrafluoroaluminate (KAlF.sub.4) is useful as a flux used for brazing the aluminum material. The KAlF.sub.4 is a complex salt obtained by mixing and melting equimolar amounts of potassium fluoride (KF) and aluminum fluoride (AlF.sub.3). Where this is used as a flux, it is pulverized and applied to brazing desired parts. This flux melts at a temperature of about 570.degree. C., and removes or melts oxides present on the surface of the aluminum material, but it is inherently inactive to aluminum metal and therefore has excellent properties as a brazing flux. Said flux, even if remaining on the brazed part after brazing, is substantially insoluble in water and hence does not corrode the aluminum material.
Accordingly, it has advantages washing after brazing, otherwise required when a chloride flux is used, is unnecessary. Painting may also be provided without giving any special treatment to products, etc.
However, this flux has a drawback that the method of supplying said flux to the brazing part is complicated. In other words, as described above, the flux is a powder mainly composed of potassium tetrafluoroaluminate. As the method of applying this powder to the brazing part, there has been proposed a method which comprises firstly suspending this powder, and a powdered brazing alloy if needed, in water, immersing therein an assembly composed of the aluminum material before brazing, then picking up said assembly from the suspension after sufficient immersion, and drying,thereby depositing the flux,or the flux and the brazing alloy,on the brazing part.
Thereafter, said assembly is heated to a predetermined temperature to melt the flux and the brazing alloy, whereby the brazing alloy penetrates through the brazing part and, when cooled, said brazing part is joined by the brazing alloy. This method, however, has a disadvantage that, in order to supply the required amounts of the flux and the brazing alloy to the brazing part, it is necessary to finely adjust the amounts of the flux and the brazing alloy in the aqueous suspension, the pick-up speed and pick-up mode after immersion of the assembly, etc.
Further, since aluminum products obtained using the above-described brazing method, for example, radiators for automobiles, are subject to pitting corrosion due to chlorine resulting from deicing salt, sea salt, etc., and hence tend to form penetrated holes, it is necessary to prevent this. For that purpose, it is required to form a zinc or zinc alloy layer for forming a pitting corrosion inhibiting layer on the surface of the aluminum material to be used, beforehand in a separate step. That is, in order to produce aluminum products by brazing, there was a problem that a number of processing steps were required.